As shown in FIG. 1, a hysteretic mode LED driver 10 is a device for providing a driving current IL for an LED 12. In the hysteretic mode LED driver 10, a power stage 13 provides the driving current IL for the LED 12 responsive to a control signal Sc, a sensor 14 senses the driving current IL to generate a sensing signal Ic, and according to the sensing signal Ic and a reference signal Vref1 provided by a signal source 16, a hysteretic comparing circuit 17 controls the duty of the control signal Sc to control the peak value and valley value, and hence the average value, of the driving current IL. The power stage 13 includes an inductor L, a power switch MN and a diode D1. The inductor L is connected between the cathode of the LED 12 and the power switch MN, and the diode D1 is connected between the inductor L and a power input terminal VIN. The hysteretic comparing circuit 17 includes a hysteresis controller 20 to generate a sensing signal Vcomp responsive to the sensing signal Ic, and a comparator 18 to compare the sensing signal Vcomp with the reference signal Vref1 to generate the control signal Sc to switch the power switch MN and thereby control the average value of the driving current IL. The hysteresis controller 20 includes serially connected resistors R1 and R2 and a switch M1 parallel connected to the resistor R1 and controlled by the control signal Sc. FIG. 2 is a waveform diagram of the hysteretic mode LED driver 10, in which waveform 22 represents the driving current IL, waveform 24 represents the reference signal Vref1, and waveform 26 represents the sensing signal Vcomp. Referring to FIGS. 1 and 2, at beginning, the driving current IL is zero, and so are the sensing signals Ic and Vcomp. At this state, the reference signal Vref1 is higher than the sensing signal Vcomp, so the control signal Sc is high and thus turns on the switches MN and M1. While the power switch MN is on, the driving current IL increases and the sensing signals Ic and Vcomp rise along with the driving current IL. Once the sensing signal Vcomp crosses over the reference signal Vref1, as shown at time t1, the control signal Sc is switched to low and thus turns off the switches MN and M1. At the moment that the switch M1 is turned off, even though the sensing signal Ic remains unchanged, the resistance of the hysteresis controller 20 changes from R2 to R1+R2 and as a result, the sensing signal Vcomp is raised by a hysteretic band and thus keeps the control signal Sc at low. On the other hand, during the power switch MN is off, the driving current IL gradually falls down as it flows through the diode D1 to discharge slowly, and therefore the sensing signal Vcomp gradually decreases. Once the sensing signal Vcomp drops below the reference signal Vref1, as shown at time t2, the control signal Sc is switched to high and thus turns on the switches MN and M1 again. At the moment that the switch M1 is turned on, the resistance of the hysteresis controller 20 changes from R1+R2 to R2, thereby pulling down the sensing signal Vcomp by a hysteretic band, and the driving current IL begins to increases again. Since the resistance of the hysteresis controller 20 is switched by switching the switch M1, the width of the hysteretic band is determined by the resistance of the resistor R1.
Based on the same principle, as shown in FIG. 3, in another hysteretic mode LED driver 30, the control is carried out by shifting the reference signal Vref1 instead of the sensing signal Vcomp. In addition to the power stage 13, the hysteretic mode LED driver 30 further includes a sensor 32, a hysteretic comparing circuit 33 and a signal source 36. Similar to that shown in FIG. 1, the sensor 32 senses the driving current IL to generate the sensing signal Ic; however, the sensing signal Ic flows through a resistor R4 to generate the sensing signal Vcomp. In the hysteretic comparing circuit 33, the hysteresis controller 20 generates the reference signal Vref1 with a reference signal Iref provided by a signal source 36, the comparator 18 compares the sensing signal Vcomp with the reference signal Vref1 to generate the control signal Sc to switch the power switch MN to control the average value of the driving current IL, and an inverter 34 generates a control signal Sc′ by inverting the control signal Sc to control the switch M1 and thereby shift the reference signal Vref1 by a hysteretic band. FIG. 4 is a waveform diagram of the hysteretic mode LED driver 30, in which waveform 38 represents the driving current IL, waveform 40 represents the reference signal Vref1, and waveform 42 represents the sensing signal Vcomp. Referring to FIGS. 3 and 4, at time t3, the sensing signal Vcomp becomes lower than the reference signal Vref1 and thus the comparator 18 turns on the control signal Sc to switch the power switch MN on and the switch M1 off. As soon as the switch M1 is turned off, the resistance of the hysteresis controller 20 changes from R2 to R1+R2 and thereby the reference signal Vref1 is lifted up by a hysteretic band, as shown by the waveform 40. On the other hand, during the power switch MN is on, the driving current IL increases, and the sensing signal Vcomp increases along with the driving current IL, as shown by the waveforms 38 and 42. Then, at time t4, the sensing signal Vcomp crosses over the reference signal Vref1, so the control signal Sc returns to low and thus turns the power switch MN off and the switch M1 on. At the moment that the switch M1 is turned on, the resistance of the hysteresis controller 20 changes from R1+R2 to R2, thereby pulling down the reference signal Vref1 by a hysteretic band, as shown by the waveform 40. During the power switch MN is off, the driving current IL gradually decreases as it flows through the diode D1 to discharge slowly, and therefore the sensing signal Vcomp decreases along with the driving current IL, as shown by the waveforms 38 and 42.
Although the hysteretic mode LED drivers 10 and 30 have the advantages of simple circuitry and fast response, the comparator 18 usually has delay response in the hysteretic mode, resulting in that the actual time point of response comes later than it is supposed to, and thus leading to an error in the average value of the driving current IL. In particular, the greater the slope of the sensing signal Ic is, the greater the error will be. This drawback is inherent in all the hysteretic mode LED drivers and is further explained with reference to FIG. 5, in which waveform 50 represents the actual driving current IL, waveform 52 represents the average value of the actual driving current IL, waveform 54 represents the reference signal Vref1, waveform 56 represents the actual sensing signal Vcomp, waveform 58 represents the ideal driving current IL, waveform 60 represents the average value of the ideal driving current IL, and waveform 62 represents the ideal sensing signal Vcomp. Ideally, as shown by the waveforms 58 and 62, when the sensing signal Vcomp rises above the reference signal Vref1, the power switch MN should be turned off instantly, thus allowing the driving current IL to decrease, and when the sensing signal Vcomp falls below the reference signal Vref1, the power switch MN should be turned on immediately so that the driving current IL begins to increase. However, due to the delay response of the comparator 18, the power switch MN will not be turned off until some time after the sensing signal Vcomp crosses over the reference signal Vref1, as shown by the waveform 56, and hence the actual driving current IL will have a higher peak value than the ideal driving current IL, as shown by the waveforms 50 and 58, resulting in a higher actual average current than the ideal average current, as shown by the waveforms 52 and 60.